The invention relates to an intermediate circuit for connecting a fuel cell stack to a load bus, a fuel cell system having the intermediate circuit, and a method for controlling the intermediate circuit.
Fuel cell stacks are an innovative, environmentally friendly alternative to energy generation, and as an energy source for vehicle propulsion systems, have the potential for at least partially replacing internal combustion engines, which have been customary in the past.
However, for operation of fuel cell stacks in vehicles consideration must be given to the constraints of this technology. In particular for starting up and switching off the fuel cell stack, it is necessary to disconnect the fuel cell stack from the load, since the fuel cell stack is able to produce only a reduced amount of power in these operating modes. For these reasons it is customary to disconnect the fuel cell stack from the load in specific operating modes, using a switching element.
For example, German patent document DE 199 54 306 A1 discloses a device for generating electrical power using a fuel cell in a vehicle, in which the fuel cell and drive motor are separably connected using a mechanical ground contact.
European patent document EP 1 375 239 A2 proposes a method and a device for controlling a fuel cell system. For supplying the load consumers, a fuel cell stack is connected to multiple load consumers in such a way that the supply may be interrupted by a switching element. According to the description, the switching element is designed as a MOSFET or IGBT.
One object of the present invention is to provide an intermediate circuit for connecting a fuel cell stack to a load bus, a fuel cell system, and a method for controlling the intermediate circuit, so that a fuel cell stack may be switched on and off on a load bus, with particular consideration for safety requirements in order to increase the operational reliability.
The fuel cell stack may have any given design, and in particular for use in a vehicle preferably has fuel cells of the PEM design. The fuel cell stack includes two supply outputs, between which a supply voltage Ustack is applied during operation.
Associated with the load bus are two load inputs, to which a load bus voltage UHV is applied during operation. Loads may be connected to the load bus, in particular a drive motor for a vehicle or other components such as heaters, lights, etc. The load bus is preferably connected to additional energy storage elements, for example an automobile battery or a SuperCap, optionally with the connection of a DC/DC converter in between. The operating voltage Ustack and/or load bus voltage UHV is/are provided as direct-current voltage, preferably with a voltage of approximately 12 V or 42 V, respectively.
The intermediate circuit according to the invention has a first electronic switching element which is connected between the fuel cell stack and the load bus such that the fuel cell stack may be switched on or off on the load bus. For this purpose an electrical connecting line is preferably switched through or blocked between one of the supply outputs and one of the load inputs. In alternative embodiments two electrical connecting lines, each connected between one of the supply outputs and one of the load inputs, is switched through or blocked.
According to the invention, at least one additional electronic switching element is provided in the intermediate circuit, so that in particular when the first switching element is closed, the fuel cell stack and/or the load bus may be short-circuited, whereby in such a short circuit the supply outputs are interconnected and/or the load inputs are interconnected, in particular connected with low-resistance conduction.
The invention is directed to the consideration that by using such an additional electronic switching element, safety functions in the intermediate circuit may be implemented effectively and economically.
In one preferred embodiment, the at least one additional electronic switching element includes a second electronic switching element or is designed as a second electronic switching element. The second electronic switching element is provided in the intermediate circuit in such a way that when the first switching element is opened the second switching element is able to short-circuit the fuel cell stack, in particular the supply outputs. As explained in detail below, measurement functions and thus monitoring and safety functions may be implemented using the second switching element. The second switching element also allows improved “heating up” of the fuel cell stack in the start-up phase.
In a further preferred embodiment, the at least one additional electronic switching element includes a third electronic switching element or is designed as a third electronic switching element. The third switching element is provided in such a way that when the first switching element of the load bus is open, the load bus, in particular the load inputs, may be short-circuited. The advantage of the third switching element is that the load bus may be rapidly discharged in the event of a hazard or for service, resulting in a safe state. The short circuit in particular ensures that the remaining load bus voltage is a safe low voltage, for example, less than 60 V.
One, several, or all of the electronic switching elements is/are preferably implemented as a semiconductor component, in particular as a transistor, IGBT (insulated gate bipolar transistor), or MOSFET. The use of electronic instead of mechanical switching elements for switching the intermediate circuit has the advantage that installation space and costs may be conserved while at the same time realizing higher reliability and longer service lives of the switching elements. Solely for the sake of completeness, it is noted that the terms such as “opening” or “closing” of the switching element are preferably equivalent to low-resistance switching or high-resistance switching, respectively, of the switching element.
In one particularly preferred embodiment, the first, second, and third switching elements are connected to one another in a PI configuration. The term “PT configuration” refers to a system based on the Greek letter pi. In the present context, the PI configuration is preferably implemented in such a way that the first switching element is connected in series between a first supply output and a first load input. On the fuel cell stack side, the input of the first switching element may be connected in an electrically conductive manner to a connecting line between the second supply output and the second load input via the second switching element. On the load bus side, the output of the first switching element may be connected in an electrically conductive manner to the same connecting line. Due to the parallel connection of the second and third switching elements the PI configuration has a redundant design; the redundancy may be further improved by connecting additional switching elements in parallel. The additional switching elements are connected either analogously to the second switching element or analogously to the third switching element.
In one particularly advantageous embodiment, the first switching element is designed and/or connected for disconnecting the fuel cell stack and load bus without reverse current flow. The first switching element preferably has a valve effect, and allows the current to flow from the fuel cell stack only in the direction of the load bus. In this manner reverse current flow is prevented as a result of the power converter function of the first switching element. This design allows the intermediate circuit to operate at a load bus voltage UHV which in particular has a higher value than the supply voltage Ustack, since current flow from the load bus is prevented in the direction of the fuel cell stack. The described operating mode is present, for example, in the start-up process or when the fuel cell stack is switched off. Another application is for the case in which the fuel cell stack is to be short-circuited while the supply bus still has voltage applied.
In one particularly simple and effective optional embodiment, the first switching element is provided only singly in the intermediate circuit, thereby minimizing the ohmic loss during operation as a result of the number of switching elements.
One advantageous embodiment of the intermediate circuit includes a first control device (a driver device) for the second switching element, while the first control device is designed for clocked (pulse width modulated) control of the second switching element. This design allows, for example, clocked short circuiting of the fuel cell stack in order to operate the fuel cell stack under load or to discharge same. This type of loading of the fuel cell stack may be advantageously used in particular in the start-up process, or also when idling after start-up.
In a further advantageous embodiment, a second control device (a driver device) is provided which controls the third switching element. The second control device is designed and/or connected in such a way that in the event of malfunction (in particular, a voltage drop in the load bus), the third switching element is automatically or independently closed and the load bus is short-circuited.
The invention further relates to a fuel cell system which includes the fuel cell stack, the load bus, and the described and claimed intermediate circuit. In this regard it is disclosed that the intermediate circuit is also suitable for the connection of other electrical energy storage elements or energy converters. As energy storage elements or energy converters, circuits may be used in particular with fuel cells, and/or double-layer capacitors, and/or batteries, and/or electrical machines (motors/generators), and/or solar cells.
The invention further relates to a method for operating the described intermediate circuit or fuel cell system, which optionally includes one or more of the following operating modes in any given selection and combination: start-up process, switch-off process, first emergency shutdown mode, second emergency shutdown mode, and/or self-diagnostic mode.
Particularly advantageous are embodiments in which the second switching element is controlled in a clocked manner by clocked short-circuiting of the fuel cell stack, in particular the supply outputs, in order to discharge the fuel cell stack or control the discharge current. Such discharge or control is preferably carried out during a start-up process for the fuel cell stack while the first switching element is still open. The fuel cell stack is placed under load as a result of the clocked operation, while at the same time the current flowing through the second switching element is limited and controlled by the clock ratio. Clocked operation of the second switching element may also be advantageously used after the start-up process in order to place the fuel cell stack under load. The clocked operation is preferably carried out by pulse width modulation of the control signal sent from the control device to the second switching element, thereby subjecting the control signal to pulse width modulation (PWM).
The described invention may be used in any fuel cell system to disconnect the fuel cell stack from the load bus (HV bus). In addition, for opening the intermediate circuit the load bus is short-circuited, thus limiting the load bus voltage to a safe level and meeting the applicable safety requirements. The use of electronic instead of mechanical switching elements allows more flexible control in addition to the other advantages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.